Three Papers by Harlow Shapley
(1) Harlow Shapley and Heber D. Curtis. "The Scale of the Universe." Separate printing, Bulletin of the National Research Council, Vol 2, part 3, number 11, May 1921. Pp 171-217. Original printed wrappers. A Good (and perhaps a Very Good) copy, pretty well read.
Full text of the report here.
A good explanation of the debate between Curtis and Shapley lives at the NASA site here:
"Although the `Great Debate' is important to different people for different reasons, it is a clear example of humanity once again striving to find its place within the cosmic order. In the debate, Shapley and Curtis truly argued over the ``Scale of the Universe," as the debate's title suggests. Curtis argued that the Universe is composed of many galaxies like our own, which had been identified by astronomers of his time as ``spiral nebulae". Shapley argued that these ``spiral nebulae" were just nearby gas clouds, and that the Universe was composed of only one big Galaxy. In Shapley's model, our Sun was far from the center of this Great Universe/Galaxy. In contrast, Curtis placed our Sun near the center of our relatively small Galaxy. Although the fine points of the debate were more numerous and more complicated, each scientist disagreed with the other on these crucial points."
"A partial resolution of the debate came in the mid-1920's. Using the 100 inch Hooker Telescope at Mount Wilson, then the largest telescope in the world, astronomer Edwin Hubble identified Cepheid variable stars in the Andromeda Galaxy (M31) . These stars allowed Hubble to show that the distance to M31 was greater than even Shapley's proposed extent of our Milky Way galaxy. Therefore M31 was a galaxy much like our own. In the 1930s, the further discovery of interstellar absorption combined with an increased understanding of the distances and distribution of globular clusters ultimately led to the acceptance that the size of our Milky Way Galaxy had indeed been seriously underestimated and that the Sun was not close to the center. Therefore, Shapley was proved more correct about the size of our Galaxy and the Sun's location in it, but Curtis was proved correct that our Universe was composed of many more galaxies, and that ``spiral nebulae" were indeed galaxies just like our own."
And what about the "great" part of this debate? Well, it was, it just wasn't very well known outside academic circles, and certainly didn't take hold of the popular imagination of the time. So it really was a great debate--it was just quiet.
(2) On the Distribution of Galaxies. Pp 389-393.
(3) Note on the Problem of the Expanding Universe. Pp 148-154.
Both offprints from The Proceedings of the National Academy of Sciences, 19/4 (April 1933) and 24/3 (1938), respectively. Both very good copies.
$300 for the three.